Designers of integrated circuits constantly strive to make the devices smaller so that more active devices can be placed on a single integrated circuit and so that the integrated circuit will have a lower cost. As devices become smaller, it becomes increasingly more difficult to prevent the formation of parasitic devices. One example of a parasitic device is the formation of a parasitic bipolar transistor between the source and drain of a field effect transistor with the bulk substrate being the remaining terminal of the bipolar transistor. In CMOS architectures, adjoining field effect devices can both create parasitic bipolar transistors in such a manner that a parasitic silicon controlled rectifier is formed. This condition is referred to as latch up and can destroy the normal operation of the CMOS system.
After an integrated circuit has been completed, the system is often tested using a burn-in process. During thus burn-in process, a higher than normal voltage is applied to the terminals which act as supply voltages within the: circuit. The circuit is then exercised at the higher than normal operating voltage. If the device is susceptible to failure, the burn-in process can cause such failures to occur to isolate and eliminate the susceptible devices. For example, burn-in is a useful way of eliminating devices that have defective gate oxides formed within field effect: transistors. Because of the higher voltages used during burn-in, an integrated circuit is ordinarily much more susceptible to both formation of unwanted parasitic devices and in some cases, latch up of the device.
Prior systems have attempted to address the latch up problem by placing a negative voltage on the substrate of the integrated circuit. This back-gate bias voltage makes it more difficult to forward bias the base emitter junction of a parasitic bipolar transistor and, thus, helps to prevent the latch up condition. However, a negative substrate bias can reduce the operating efficiency of the integrated circuit during normal operations after burn-in processing is complete.